Other ZooplanktonL G Rudstam, Cornell University, Ithaca, NY, USA

ã 2009 Elsevier Inc. All rights reserved.

Other Zooplankton

Several other organisms in addition to copepods,cladocerans, and rotifers can be considered partof the zooplankton. This is a diverse group thatincludes pericarid crustaceans such as mysid shrimpsand amphipods, both larval and adult insects –particularly larvae of the phantom midges in thegenus Chaoborus, water mites, fairy shrimps, fresh-water jellyfish, and planktonic mussel larvae (Dreis-sena veligers). Several of these species are relativelylarge predators or omnivores (5–30mm), can be abun-dant, and do strongly affect the population dynamicsof their prey. Because of their large size, they are alsosensitive to predation by fish. In contrast, veligers aresmall filter feeders (0.07–0.2mm).

Mysid Shrimps

Mysid shrimps, or opossum shrimps, are membersof a mostly marine order (Mysidacea). There aresome 30 species occurring in freshwater lakes andrivers and the group has a worldwide distribution.Lacustrine species are primarily of the genus Mysisand Neomysis, but there are several Ponto-Caspianspecies that are spreading to Europe and NorthAmerica. For example, Hemimysis anomala invadedEuropean lakes from the Ponto-Caspian region in thelast century and reached North America in 2006. Inmarine systems, mysids are primarily found in ben-thic and nearshore habitats and they are an importantcomponent of estuarine food webs.Mysis relicta (Figure 1) is the most studied mysid

shrimp. It is a glacial relict with a circumpolar distri-bution in deep cold lakes of the northern hemisphere.Mortality increases when Mysis has been held long-term in temperatures above 13 �C. Their absence fromdeep cold lakes within a few kilometers of their natu-ral range – areas covered by ice lakes after the lastglaciation – is a testimony to their poor natural dis-persal ability. TheMysis relicta species group consistsof four species, one in North America (M. diluviana)and three in Eurasia (M. relicta, M. segerstralei,M. salemaai). Members of the genus Mysis also occurin marine and brackish water and in the Caspian Seawhere three morphologically distinct but closelyrelated species coexist. The abundance of mysids canbe high and their biomass can exceed that of plankti-vorous fish. Density and biomass vary seasonally

and densities over 500m�2 and a biomass of 10 gww m�2 are not uncommon; peak densities over1000m�2 have been reported. Mysids do not havegills; oxygen is taken up through the thin carapace.They are rarely found in lakes without oxygenatedhypolimnion in the summer.

Mysis relicta can reach 25–30mm, grow slowlyand produce one or two clutches of 10–50 young ina lifetime. Growth rates and therefore generationtime depend on lake productivity. Growth ratesincrease in more productive lakes and range from0.2mm month�1 in Lake Tahoe to 1.5mm month�1

in mesotrophic lakes. Corresponding generationtimes are 1 year in productive lakes and 2–3 years inoligotrophic lakes; up to 4 years has been reportedfrom ultra-oligotrophic Lake Tahoe. Some lakeshave both a 1- and a 2-year generation time withslow growing individuals taking 2 years to mature.One Swedish population with a 2-year life cycleswitched to a 1- year life cycle for several yearswhen transplanted to a more productive lake. Theembryos are carried by the female in a brood pouch(marsupium) for several months; hence the nameopossum shrimp. Males die after copulation, butfemales may molt and produce a second clutch.The young ones are released at a length of 2–3mmand at that size they appear similar to the adults.There are no free swimming naupliar stages.A springtime release is common, but some popula-tions also release young in the fall or throughout theyear, and therefore consist of overlapping genera-tions. With such a long generation time and smallbrood size, mysid populations are sensitive to highmortality rates and would therefore not persist insystems without a refuge from fish predation.A population with a 2-year generation time produc-ing 30 young once per female requires a 25% annualsurvival rate to persist in the system. This is a veryhigh survival rate compared with, for example, fish,which can have a first year survival of 1% or less.

Mysis performs remarkable diel migrations at duskand dawn from their daytime refuge in dark, deepwater to the meta- or epilimnion, where they feedon zooplankton or algae. These migrations can beover 100m long (Figure 2) and migration speed canexceed 1–2m min�1. During the day, part of themysid population may remain in the water columnif the lake is deep enough. Some animals stay on thebottom day and night. Their migration is limited by

667

668 Zooplankton _ Other Zooplankton

temperatures above 12–16 �C and light levels above10�4 lux, light levels that limit fish visual feeding.Mysis have one type of photo-pigment with peaksensitivity between 490–540 nm (adapted to the

Figure 1 Adult female Mysis relicta (top) and adult

female Macrohectopus branickii (bottom). Both animals arearound 20mm long. Adapted with permission from Rudstam LG,

Melnik NG, and Shubenkov SG (1998) Invertebrate predators in

pelagic food webs: Similarities betweenMacrohectopus branickii

(Crustacea: Amphipoda) in Lake Baikal and Mysis relicta(Crustacea: Mysidaceae) in Lake Ontario. Siberian Journal of

Ecology 5: 429–434.

Figure 2 Downwards migration of Mysis relicta in Lake Ontario, Juechosounder. Vertical lines represents 30min time step. Sunrise at 0

The mysid layer moved 100m in 1.5 hours, for a migration speed of 1

10m, 5.1m�3 in 10–30m, 0.9m�3 in 30–70m, and 0.3m�3 in 70–120m

layer, possibly small mysids, are also visible in the water column afte

local light environment). They are not sensitive tolong wavelength red light. Moonlight will affecttheir nighttime distributions and they are tens ofmeters deeper during full moon nights comparedwith new moon nights. Larger mysids from LakeOntario have a strong preference for temperaturesbetween 6 �C and 8 �C; smaller mysids have less pro-nounced temperature preferences but avoid tempera-tures above 14 �C. The vertical distribution of mysidsis largely predictable from their response to tempera-ture and light. There is a cost to residing at low tem-peratures of 4 �C during the day, asmysid developmenttime is slower at 4 �C than at 8 �C.

Mysids are omnivores and capable of both filter-feeding and raptorial feeding. A feeding current iscreated by the thoracic legs, which also provide forregular swimming motion. Diatoms are consumedduring spring and to a lesser extent during the falldiatom blooms and algae can then be a major com-ponent of the diet. Small mysids are more herbivo-rous than large mysids. They also feed on benthicprey, detritus, and sediment during the day. By feed-ing in both the benthic and pelagic layer, mysidscan be a vector for redistribution of nutrients andcontaminants between these two subsystems. Zoo-plankton typically dominates the diet. Zooplanktonare captured by fast movement towards the prey. Thesize of prey captured increases with the size of themysids, and larger mysids can feed on amphipods andsmaller conspecifics.

0

20

40

60

80

100

120

0

Temperature �C

302010

ly 31, 1995, from 02:30 to 05:40 observed with a 420 kHz4:00. Bottom depth 130m. The temperature profile is also given.

m min�1. Net samples prior to 02:30 showed no mysids in the top

depth. Some fish and what appear to be a shallower invertebrate

r the main mysid layer has descended.

Zooplankton _ Other Zooplankton 669

Small mysids actively avoid larger individuals,potentially to reduce predation risk. Feeding rate ofmysids decreases with mysid density in the labora-tory, indicating interference among conspecifics.Mysids consume more cladocerans than copepodsbecause copepods are more efficient at avoiding cap-ture. Representative hard parts of crustaceans, roti-fers, and diatoms can be identified in the foregut.Mysid feeding rates decline in the presence of fish orfish kairomones. Clearance rates from experimentsvary with prey species and range from 0.05 l hour�1

when feeding on large copepods to 1 l hour�1 whenfeeding on cladocerans. Short-term specific feedingrates can be as high as 25% of body weight per day,although most reports from field and long-term labo-ratory experiments indicate values of 5–10% per day.Assimilation efficiency is high (80–90%) when feedingon zooplankton or chironomids, lower when feed-ing on detritus. Gross growth efficiency for animalsfeeding on zooplankton is between 7% and 29%. Atleast one species (Mysis stenolepis) is able to assimilateraw sterilized cellulose with an efficiency of 30–50%.Mysids are an important food resource for fish and

several species rely primarily on mysid shrimps.Mysids have a fast avoidance reaction to fish attacks(80 body length s�1) through tail flipping and canoften avoid capture. In lakes with native mysid popu-lations, fish have evolved adaptations for migratingwith their mysid prey, such as reduced swim bladders,high fat content and large fins. Mysids are rich inessential fatty acids and may be even more importantfor fish than suggested by their quantitative propor-tion in the diet. Polyunsaturated fatty acids commonin mysids are important for over-winter survival ofseveral fish species.Mysis relicta shrimps were introduced in 1949 to

Kootenay Lake, BC, Canada and are believed to be atleast partly responsible for a large increase in growthrate of kokanee salmon (Oncorhynchus nerka) andthe spectacular fishery that developed after the intro-duction. After this reported initial success, mysidswere introduced to many lakes and reservoirs inwestern USA, Canada, and Scandinavia to increasefish growth and production. However, results werenot often as intended, as mysid predation causeddeclines in cladocerans, in particular Daphnia. SinceDaphnia is an important prey of juvenile stages ofseveral fish species, these fish species often declinedafter mysid introductions. Even in Kootenay Lake,kokanee declined in the 1970s although it is uncertainto what degree this was caused by mysids. Althoughthe consequences of mysid introductions were oftendetrimental to established fisheries, these introductionsdid show that mysids can affect food web dynamics.

Indeed, mysids can be quantitatively more importantzooplanktivores than fish. In Flathead Lake,Montana,the introduction of mysids led to a marked decline inkokanee and subsequent decline in bald eagles andother birds and mammals that fed on the spawningrun of the kokanee. There is now a moratorium onmysid introductions.

Members of the Mysis relicta species complex arenot the only mysids in freshwater. Several species ofNeomysis are present in lakes and estuaries. Thesespecies can be found in warmer waters than Mysisrelicta, are omnivores, major predators on zooplank-ton, and perform diel migrations. Neomysis sp. aresmaller (10–15mm) than Mysis sp., and may havetwo or more generations per year.Neomysis mercedisis a major zooplanktivore in Lake Washington andother lakes in the Pacific North-West of NorthAmerica. Neomysis integer is a European species pri-marily found in brackish lakes and estuaries as well asin the Baltic Sea. In shallow brackish lakes inDenmark, Neomysis integer is abundant only whenthe fish community is dominated by small fish, such assticklebacks. When abundant, this species is an impor-tant predator on zooplankton and an important preyfor fish, and increases in Neomysis following fishremoval have in some cases negated the intended effectof increases in Daphnia grazing. Some of the highestmysid densities (11 000m�2, 40g wwm�2 assuming a15% dry weight) have been reported for Neomysisintermedia in hyper-eutrophic Lake Kasumigaura,Japan. Tenagomysis chiltoni is abundant in turbid,shallow New Zealand lakes. Littoral mysids, such asHemimysis anomala and Limnomysis benedeni can beabundant in both small and large lakes, and will thenalso affect zooplankton species composition.

Other Crustaceans

In some lakes, a similar life history strategy to mysidshave evolved among amphipods. For example, theamphipod Macrohectopus branickii is abundant inthe open water of Lake Baikal and is similar inappearance to mysids (Figure 1). Females can be upto 38mm long although they mature at 16–20mm.Brood size increases with female length from 90 to400 eggs per female. Males mature at smaller size(5–6mm). The species is an omnivore that migratesover 100 meters each night to feed in the surfacewater. As inMysis relicta, mortality ofMacrohectopusincreases over 12–13 �C; they avoid light levels above10�4 lux, and become more predatory as they growlarger. Available data suggest that Macrochectopushas a generation time similar to Mysis relicta makingthis species similarly sensitive to predation. The

670 Zooplankton _ Other Zooplankton

biomass ofMacrohectopus (4–10g wwm�2) is as highor higher than the biomass ofMysis in the Great Lakes(2–5g ww m�2 in Lake Ontario). This species isendemic to Lake Baikal and may be the amphipodspecies with the most extreme ecological divergencefrom its closest relative. There are no mysids in LakeBaikal. The remarkable similarity in the ecology ofMysis and Macrohectopus is likely due to the similarselective pressures on large migrating invertebratepredators in deep cold lakes. There is a cost to spendingthe day in deep, cold water both because there is lessfood in that area, and because cold water decreasesmetabolism, increases digestion time, and decreasesgrowth (when food is plentiful). Slower growth leadsto longer generation times, and therefore to popula-tions that are even more sensitive to predation thanif they resided in the warmer epilimnion both dayand night. This will further increase the selectivepressure for predator avoidance through migrations.Amphipods are an important component of the lim-netic zooplankton in other lakes as well. For example,the omnivorous amphipod Jesogammarus annandalenis abundant and performs nocturnal migrations intothe water column of Lake Biwa, Japan.In the East African Great Lakes a major compo-

nent of the zooplankton are decapod shrimpsin the families Atyidae and Palaemonidae. Theup to 25mm-long atyid shrimp Caridina niloticareach densities of 1000m�2 in Lake Victoria. Theyincreased in abundance after the introduction ofthe Nile perch (Lates niloticus), likely an indirecteffect of the decline in haplochromine cichlidsassociated with the increase in Nile perch. Cari-dina feed on detritus and phytoplankton and per-form diel migrations to feed at the surface at night.Only small individuals remain in the upper wateralso during the day.Fairy shrimps (Anostraca) can be very common

in fish-less ponds and salt lakes. Some are preda-tors but most feed by filtering algae and detritus.The brine shrimps (Artemia sp.) are found world-wide in high salinity lakes and in ponds that aretoo saline for fish. This is one of the few zooplank-ton that are harvested commercially. Large quanti-ties of Artemia resting eggs are used in the fishculture industry. These resting eggs can be storeddry for long periods of time, and made to hatch byhigh temperature, salinity and light.

Figure 3 Fourth instar larva (top), pupa (left), and adult (right) of

Chaoborus punctipennis. Adapted from Johannsen OA (1934)

Aquatic Diptera. Part I. Nemocera, exclusive of Chironomidaeand Ceratopogonidae. Memoirs of the Cornell University

Agricultural Experimental Station 164: 1–70.

Chaoborus Larvae – the Phantom Midge

The phantom midge larvae of the genus Chaoborusare a major component of the planktonic commu-nities of many lakes. There are about 50 extant spe-cies in 6 genera and 2 sub-families in the family

Chaoboridae. They are found in lakes worldwidefrom the tropics to the arctic regions and occur insome of the largest lakes in the world (e.g., LakesVictoria and Malawi) as well as in small ponds. Theadult insects are medium-sized, non-biting, live forabout a week and deposit several hundred eggs inrafts on the water surface. Eggs hatch in a few days.There are four larval instars. The first two are lim-netic and last a couple of weeks. The third instaroccurs both in the open water and burrowing in thesediment. The fourth instar (Figure 3) typically repre-sents the largest biomass and has the largest effect onits prey. This stage can last more than one year and isoften the stage that overwinters. The pupae stage(Figure 3) lasts a few days to 2weeks depending ontemperature. Depending on whether the generationoverwinters or not, the life cycle duration is a coupleof months to more than half a year. Some species havea 1- or even 2-year life cycle.

When Chaoborus larvae burrow into the sedimentthey assume a vertical anterior-end up S-shaped posi-tion and do not maintain a tube connection to theoverlying water. Because they burrow into the anoxiclayer, they are likely tolerant to hydrogen sulfidepresent in those layers. They also feed on benthicprey such as oligochaetes and benthic harpacticoidcopepods.

Different species vary in their sensitivity to fishpredation. Larvae of most species are transparentand move little except during migration. Swimmingis a jerky motion produced by flexing the body back

Zooplankton _ Other Zooplankton 671

and forth. The larva has two paired air sacks that areused to maintain buoyancy without the need for con-spicuous swimming movements. The eye is the mostvisible part of the body and larger eyes as well asincreased stomach content increase predation risk.The diel migration is a predator avoidance behavior.Predation by fish on Chaoborus is more intense onthird and fourth instar larvae. These are also theinstars that migrate. Most fish will not enter waterwith oxygen levels below 1–2mg l�1 and the lowoxygen levels in the sediment or hypolimnion there-fore provide a refuge from fish predation during theday. The migration is triggered by decreasing lightlevels and the animals move using gravity as a cue –downwards in high light levels and upwards at lowlight levels. When fish are abundant, Chaoboruslarvae initiate migration at a lower light level. Notall individuals in a population migrate; the propor-tion that migrates varies with season and is highest insummer and lowest in winter. Chaoborus is adaptedto low oxygen conditions but may incur an oxygendebt during the day. Pupae that spend part of theday in anoxic conditions develop slower than pupaethat remain in oxic conditions. Pupae also oftenmigrate into the water at night, probably to speedup development time in higher oxygen concentra-tions. Migration distance is typically tens of meters,but in Lake Malawi, Chaoborus fourth instar migratefrom daytime depths of 150–200m to the surfacewaters each night. Not all Chaoborus species havelarvae that migrate. There are larger species commonin fishless lakes that do not. Chaoborus will ceasemigration after some time if fish are removed, butthis change in behavior is slower than the inductionof migration when fish are added to a lake.Chaoborus larvae are ambush predators that can

be motionless in the water owing to the buoyancyprovided by the air sacks. When a prey swims closeenough to be detected, Chaoborus strike and capturethe prey with the modified prehensile antennae andmandibles. This strategy makes Chaoborus equallyeffective in light and dark and at catching copepodsand cladocerans, which is unusual for zooplankti-vores. The first and second instars feed on rotifersand larger algae such as dinoflagellates, whereas thethird and fourth instars feed on cladocerans and cope-pods as well as other dipterans and benthic organ-isms. Most studies show a selection for copepods overcladocerans, likely the result of differences in theirability to handle different prey and on the swimmingspeed of the prey – more active prey will be encoun-tered more often. Prey size increase with the size ofthe larvae – the width of the prey is important, not thelength. Well-fed Chaoborus are more selective as towhich prey to attack. Handling of cladocerans is

more difficult when neck teeth, elongated tail spines,and helmets are present. Such structures are inducedinDaphnia in the presence of chemicals (kairomones)released byChaoborus. The animals also feed on smal-lerChaoborus larvae and other insect larvae. Althoughseveral species of Chaoborus co-exist in many lakes,smaller species are often depressed by larger speciesand therefore the smaller species can be more abun-dant when fish are present than when fish are absent.Chaoborus are sensitive to kairomones both from theirprey and their predators. Their activity increases in thepresence of prey kairomones and migrations increasein the presence of fish kairomones.

Chaoborus can be abundant (over 10 000m�2

reported), have high production, and feeding rates ashigh as 10% of their body weight per day or approxi-mately one crustacean per hour. Although the presenceof Chaoborus does not always affect zooplankton spe-cies composition, there is clear evidence from labora-tory enclosures and field experiments showing that thispredator can have strong effects on their prey (Table 1).But the effect varies with predator and prey size andmorphology, and can change over time owing to induc-tion of structural defenses in the prey species. In abiomanipulation experiment in a German reservoir,removal of fish caused an increase in Chaoborus fol-lowed by a decline in zooplankton, in contrast to theintended effect of increasing zooplankton abundancethrough fish removal. Chaoborus and fish can alsointeract to produce a stronger effect than when eitheris present alone because the presence ofChaoborus canaffect the ability of cladocerans to use low oxygenrefuges from fish predation.

Dreissena Veligers

The planktonic larva (veliger) of dreissenid mussels isalso considered here. Most marine bivalves have aplanktonic larval stage, but most freshwater bivalvesdo not. Dreissenids release gametes in large quantities(up to a million eggs per female) in synchronizedspawning events throughout the season, often withpeaks in June and August. Zebra mussels (Dreissenapolymorpha) have optimum temperatures for spawn-ing and veliger growth between 12 �C and 24 �C, andpH between 7.4 and 9.4. Quagga mussels (Dreissenabugensis) occur in large densities in deep, consistently4 �C cool water, and likely spawn at those tempera-tures. Following established nomenclature for marinebivalves, Ackerman and colleagues recognized thefollowing stages for dreissenids (Figure 4): (1) theegg and embryonic period, (2) the free swimmingtrochophore, (3) the D-shaped veliger (formedwhen the velum – a larval organ used for feeding

Table 1 Some case studies of the ecological effects of mysids and Chaoborus on zooplankton communities

Group, Location Peak density Lake trophic status Summary of results Source

Mysis relicta

Lake Tahoe, NV, CA 400m�2 Ultra-oligotrophic Daphnia and fisheries declined after introduction of Mysis 1Lake Ontario, USA, Canada 1100m�2 Oligotrophic Native, similar feeding rates as planktivorous fish, inferred to be important planktivore 2

Lake Michigan, USA, Canada 1000m�2 Oligotrophic Native, omnivore, temperature refuge for zooplankton limits Mysis predation in summer months 3

Flathead Lake, Montana, USA 130m�2

(average)

Oligotrophic Strong food web effects through declines in Daphnia and kokanee after introduction of Mysis.

Also affecting eagles and mammals feeding on kokanee

4

Lake Pend Oreille, Idaho, USA 1250m�2 Oligotrophic Introduced, zooplanktivory similar to fish, Mysis affect seasonal zooplankton dynamics and spatial

patterns

5, 6

Lake Hiidenvesi, Finland 175m�2 Eutrophic Native but limited by low oxygen. Chaoborus more important predator on zooplankton 7

Lake Selbusj�en, Norway 200m�2 Oligotrophic Strong negative effect on cladocerans after the introduction of Mysis 8Lake Jonsvatn, Norway 130m�2 Oligotrophic Decline of Daphnia and other zooplankton in two embayments of the lake after Mysis introduction,

less effect in the main lake

9, 10

Canadian Shield Lakes,Ontario

Varied Oligotrophic Native, comparisons among similar lakes with and without Mysis show negative effects oncladocerans in the meta and hypolimnion

11

Other mysids

Lake Fering, Denmark–

Neomysis integer

900m�2 Eutrophic, brackish Native, cladoceran seasonal dynamics due to Neomysis predation 12

Lake Kasumigaura, Japan

Neomysis intermedia

17 000m�2 Eutrophic Native, strong seasonal variability in mysids related to zooplankton seasonal dynamics 13

Lake Washington, WA,Neomysis mercedis

350m�2 Mesotrophic Native, major zooplankton predator before increase in fish (longfin smelt) 14

Muriel Lake, BC, Canada

Neomysis mercedis

85m�3 Oligotrophic Native, Neomysis abundant in low sockeye salmon years and then more important than fish as

zooplankton predator.

15

Beisbosch Reservoir,Netherlands, Hemimysis

anomala

6000m�2 Eutrophic Strong decline of all zooplankton after Hemimysis invasion 16

Chaoborus

Gwendoline Lake, BC, Canada 1000m�2 Oligotrophic Fishless lake with two Chaoborus species with 1 and 2 year life cycles.Chaoborus predation interactwith seasonal algal dynamics to produce zooplankton seasonal dynamics.

17

Temporary pond, Santa Cruz,

CA

800m�3 Eutrophic Chaoborus preferred Ceriodaphnia, and this species declines during high Chaoborus abundance 18

672

Zooplankton_O

therZooplankton

Lake Hiidenvesi, Finland 14000m�2 Eutrophic Large seasonal changes in abundance of Chaoborus causes seasonal changes in zooplankton 19Lake Iso Vakljarvi, Finland 6000m�3 Eutrophic Fish kill released Chaoborus and depressed zooplankton 20

Small experimental lakes,

Germany

Eutrophic Chaoborus increase after fish removal caused decline in Daphnia. Moderate fish abundance

beneficial to Daphnia

21

Source

1. Richards R, Goldman C, Byron E, and Levitan C (1991) The mysids and lake trout of Lake Tahoe: A 25-year history of changes in fertility, plankton, and fishery of an alpine lake. American Fisheries Society

Symposium 9: 30–38.

2. Johannsson OE, Rudstam LG, Gal G, and Mills EL (2003) Mysis relicta in Lake Ontario: Population dynamics, trophic linkages and further questions. In Munawar, M. (ed.) State of Lake Ontario (SOLO) – Past,

Present and Future, pp. 257–287. Leiden, The Netherlands: Backhuys Publishers.

3. Pothoven SA, Fahnenstiel GL, and Vanderploeg HA (2004) Spatial distribution, biomass and population dynamics of Mysis relicta in Lake Michigan. Hydrobiologia 522: 291–299.

4. Spencer CM, McClelland BR, and Stanford JA (1991) Shrimp stocking, salmon collapse, and bald eagle displacement: Cascading interaction in a food web of a large aquatic eocsystem. Bioscience 41: 14–21.

5. Chipps SR and Bennett DH (2000) Zooplanktivory and nutrient regeneration by invertebrate (Mysis relicta ) and vertebrate (Oncorhynchus nerka ) planktivores: implications for trophic interactions in oligotrophic

lakes. Transactions of the American Fisheries Society 129: 569–583.

6. Clarke LR and Bennett DH (2003) Seasonal zooplankton abundance and size fluctuations across spatial scales in Lake Pend Oreille, Idaho. Journal of Freshwater Ecology 18: 277–290.

7. Horppila J, Liljendahl-Nurminen A, Malinen T, Salonen, M, Tuomaala, A, Uusitalo L, and Vinni M (2003) Mysis relicta in a eutrophic lake: Consequences of obligatory habitat shifts. Limnology and Oceanography

48: 1214–1222.

8. Langeland A, Koksvik IJ, and Nydal A (1991) Impact of the introduction of Mysis relicta on the zooplankton and fish populations in a Norwegian lake. American Fisheries Society Symposium 9: 98–114.

9. Koksvik JI, Reinertsen H, and Langeland A (1991) Changes in plankton biomass and species composition in Lake Jonsvatn, Norway, following the establishment of Mysis relicta . American Fisheries Society

Symposium 9: 115–125.

10. Naesje TF, Saksgard R, Jensen AJ, and Sandlund OT (2003) Life history, habitat utilisation, and biomass of introduced Mysis relicta . Limnologica 33: 244–257.

11. Nero RW and Sprules WG (1986) Zooplankton species abundance and biomass in relation to occurrence of Mysis relicta (Malacostraca; Mysidacea). Canadian Journal of Fisheries and Aquatic Sciences

43: 420–434.

12. S� ndergaard M, Jeppesen E, and Aaser HF (2000) Neomysis integer in a shallow hypertrophic brackish lake: Distribution and predation by three-spined stickleback (Gasterosteus aculeatus ). Hydrobiologia

428: 151–159.

13. Hanazato T (1990) A comparison between predation effects on zooplankton communities by Neomysis and Chaoborus . Hydrobiologia 198: 33–40.

14. Chigbu P (2004) Assessment of the potential impact of the mysid shrimp, Neomysis mercedis , on Daphnia . Journal of Plankton Research 26: 295–306.

15. Hyatt KD, Ramcharan CR, McQueen DJ, and Cooper KL (2005) Trophic triangles and competition among vertebrate (Oncorhynchus nerka, Gasterosteus aculeatus ) and macroinverteb rate (Neomysis mercedis )

planktivores in Muriel Lake, British Columbia, Canada. Ecoscience 12: 11–26.

16. Ketelaars HAM, Clundert FEL-v d, Carpentier CJ, Wagenvoort AJ, and Hoogenboezem W (1999) Ecological effects of the mass occurrence of the Ponto-Caspian invader, Hemimysis anomala G.O. Sars, 1907

(Crustacea: Mysidacea), in a freshwater storage reservoir in the Netherlands, with notes on its autecology and new records. Hydrobiologia 394: 233–248.

17. Neill WE (1981) Impact of Chaoborus predation upon the structure and dynamics of a crustacean zooplankton community. Oecologia 48: 164–177.

18. Riessen HP, Sommerville JW, Chiappari C, and Gustafson D (1988) Chaoborus predation, prey vulnerability, and their effect in zooplankton communities. Canadian Journal of Fisheries and Aquatic Sciences 45:

1912–1920.

19. Liljendahl-Nurmine n A, Horppila J, Malinen T, Eloranta P, Vinni M, Alaja rvi E, and Valtonen S (2003) The supremacy of invertebrate predators over fish – Factors behind the unconventional seasonal dynamics of

cladocerans in Lake Hiidenvesi. Archiv fur Hydrobiologie 158: 75–96.

20. Rask M, Ja rvinen M, Kuoppama ki K, and Poysa H (1996) Limnological responses to the collapse of the perch population in a small lake. Annales Zoologici Fennici 33: 517–524.

21. Wissel B, Freier K, Mu ller B, Koop J, and Benndorf J (2000) Moderate planktivorous fish biomass stabilizes biomanipulation by suppressing large invertebrate predators of Daphnia . Archiv fu r Hydrobiologie

149: 177–192.

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674 Zooplankton _ Other Zooplankton

and locomotion – and the first shell has been formed),(4) the velichonca (after a second shell is secreted),(5) the pediveliger (with a foot that can producebyssal treads and is used for both swimming andcrawling), (6) the postveliger or plantigrade mussel(formed after the pediveliger has selected a site,attached itself with byssal threads and undergonemetamorphosis), and (7) the adult mussel. Dreissena

Pediveliger(>167−<300 µm)

Foot Proissoconch I

Veliconcha(120−280 µm)

Life history stages of Dreiss

Pediveliger(>167−<300 µm)

Lateral view of veliger larva of D. polymorphs

Plantigrade(>158−<500 µm)

Velum

Metamorphosis:loss of velumdissoconch

Development

Ju(>500−

Figure 4 (Top) Life cycle of Dreissena mussel with line drawings o

JD, Sim B, Nichols SJ, and Claudi R (1994) Review of the early-life h

with marine bivalves. Canadian Journal of Zoology 72: 1169–1179. (B

From the zebra mussel information system of the Army Corps of Eng

veligers can be recognized in plankton samples usingcross-polarized light. The veliger has a row of cilia onthe velum that is extended out from the shell. Thesecilia are in constant motion as the animal filters par-ticles into a feeding groove where the particles areencased in mucus and transported to the stomach.During the larval period the animal increases in sizefrom 70 to over 200 mm.

I Velumprodissoconch I

ena ploymorpha (Pallas)

D−shaped veliger(70−160 µm)

Trochophore(57−121 µm)

Embryogenesis

Fertilization

Sperm(4−9 µm)

Egg(40−96 µm)

Ventral view of veliger larva of D. polymorpha

Velum

Growth

venile<5000 µm) Adult

f their larval stages. Reproduced with permission from Ackerman

istory of zebra mussels (Dreissena polymorpha) – Comparisons

ottom) Free swimming veliger of Dreissena showing the velum.

ineers: http://el.erdc.usace.army.mil/zebra/zmis/

P

I

II

Zooplankton _ Other Zooplankton 675

Veligers can be abundant. They are, at times, thedominant plankton organisms in the St. LawrenceRiver estuary and densities of over 500000m�3

have been reported from Lake Erie. They feed on1–4mm particles, including blue-green and smallgreen algae, bacteria, protists, and detritus, andthrough direct uptake of dissolved organic carbon.Filtration rates are in the order of 10–20ml hour�1.Although they are consumed by some planktivorousfish, they are not a major component of fish diets.The presence of veligers has not been shown to affectother zooplankton groups, at least not crustaceanzooplankton. Filter feeders, such as adult zebramussels, may be their main predators.

III

0.2IV

Figure 5 A nymph of the water mite Piona rufa (Adapted

by Roger Wayman from Wolcott RH (1905)Water Mites and How

to Collect. Bulletin 11 Roger Williams Park Museum, Providence,R.I. and from Riessen HP (1982) Pelagic water mites: Their life

history and seasonal distribution in the zooplankton community

of a Canadian lake. Archiv fur Hydrobiologie Suppl 62: 410–439.

Other Animals In the Zooplankton

There are other animals that should be consideredpart of the zooplankton of inland waters. Theseinclude several species of water mites and jellyfish,and various insect larvae and adults. Water mitesand freshwater medusae are predators, and bothgroups have been shown to affect their zooplanktonprey in some lakes. Planktonic water mites occurthroughout the world. The most common are inthe genus Piona. They emerge from the eggs assmall, six-legged larvae and metamorphose to thenymphal stage after 8–28 days. These nymphs arepredators on zooplankton (Figure 5). When encoun-tering a prey, the mite will grab the prey with itslegs and tear open the body wall with its chelicerae.The soft body of the prey is predigested and thendrawn into the mouth. They select cladocerans overcopepods. The larvae do not need to parasitizeinsect larvae to develop to the nymphal stage(many other water mites require feeding to developto the nymphs) and it is not known if they arefacultative parasites or not.Freshwater medusae in the family Limnomedusae

(e.g., the 20–25mm Craspedacusta sp. in Eurasia andNorth America and the similar sized Limnocnida sp.in Africa and India) are a highly variable componentof the plankton, occurring in high numbers for severalyears followed by years of low abundance. As otherjellyfish, they have a complex life cycle with bothsessile polyp stages and an open water medusa stage(Figure 6). These predators co-occur with high abun-dance of fish as the fish do not prey on them. Medu-sae feed on copepod nauplii and copepodites andsmall cladocerans.Zooplankton do not only consist of copepods,

cladocerans, and rotifers, and some of the mainplayers in planktonic food webs are in the diversegroup ‘other zooplankton’ discussed in this chapter.

Glossary

Byssal threads – formed by mussels to attach to thesubstrate.

g ww – gram wet weight.

Kairomone – chemicals used to obtain informationon presence of prey or predators.

Marsupium – brood pouch.

Nauplius – larval stage of some crustaceans groups.

Pediveliger – 4th stage of mussel verligers, have afoot.

Planula larvae – free moving larvae of freshwaterjellyfish.

Podocyst – resting stage of a freshwater polyp.

Polyp – benthic stage of jellyfish.

Postveliger or plantigrade mussel – 1st attached stageof the mussel.

Trochophore – 1st free swimming stage of musselveligers.

Velichonca – 3rd stage of mussel veligers– after sec-ond shell is formed.

Veliger – free swimming larvae of mussels.

Immaturemedusa

Mature medusa

Fertilized egg

Medusabud

Polypbud

Polyp

Frustulebud

Frustule larva

Colony

Blastula

Planulalarva

Figure 6 Life cycle of Craspedacusta sowerbii. The small stalked polyp lives in colonies attached to a substrate (bottom,docks, plants) and reproduce asexually. Sometimes, medusae are formed that move into the water column and reproduces sexually.

Both the medusae and polyps have stinging cells that will fire on contact. Fertilized eggs develop into planula larvae, which will

settle and develop into new polyps. The polyp can contract in cold temperature into a resting stage (podocyst) that survives cold

temperatures. Podocysts and polyps may be transported by animals or plants to other water bodies. Adapted by M. Thom fromLytle CF (1982) Development of the freshwater medusa Craspedacusta sowerbii. In Harrison FW and Cowden RR (Eds.) Developmental

Biology of Freshwater Invertebrates, pp. 129–150. New York: Alan R. Liss Inc. Reproduced with permission from Terry Peard,

www.jellyfish.iup.edu.

676 Zooplankton _ Other Zooplankton

See also: Aquatic Insects – Ecology, Feeding, and LifeHistory; Benthic Invertebrate Fauna, Lakes andReservoirs; Biomanipulation of Aquatic Ecosystems;Branchiopoda (Anostraca, Notostraca, Laevicandata,Spinicaudata, Cyclestherida); Cladocera; Competitionand Predation; Copepoda; Cyclomorphosis andPhenotypic Changes; Decapoda; Diel VerticalMigration; Diptera (Biting Flies); Invasive Species;Regulators of Biotic Processes in Stream and RiverEcosystems; Role of Zooplankton in AquaticEcosystems; Trophic Dynamics in Aquatic Ecosystems.

Further Reading

Ackerman JD, Sim B, Nichols SJ, and Claudi R (1994) Review of

the early-life history of zebra mussels (Dreissena polymorpha) –Comparisons with marine bivalves.Canadian Journal of Zoology72: 1169–1179.

Berendonk TU, Barraclough TG, and Barraclough JC (2003) Phy-logenetics of pond and lake lifestyles in Chaoborusmidge larvae.

Evolution 57: 2173–2178.

Borkent A (1981) The distribution and habitat preferences of the

Chaoboridae (Culicomorpha, Diptera) of the Holarctic region.Canadian Journal of Zoology 59: 122–133.

Zooplankton _ Other Zooplankton 677

Jankowski T, Strauss T, and Ratte HT (2005) Trophic interactions

of the freshwater jellyfish Craspedacusta sowerbii. Journal ofPlankton Research 27: 811–823.

Johannsson OE, Rudstam LG, Gal G, and Mills EL (2003) Mysisrelicta in Lake Ontario: population dynamics, trophic linkages

and further questions. In: Munawar M (ed.) State of LakeOntario (SOLO) – Past, Present and Future, pp. 257–287. Lei-den, The Netherlands: Backhuys Publishers.

Lasenby DC, Northcote TG, and Furst M (1986) Theory, practice,

and effects ofMysis relicta introductions to North American andScandinavian lakes. Canadian Journal of Fisheries and AquaticSciences 43: 1227–1284.

Mauchline J (1980) The biology and mysids and euphausiids. In:

Blaxter JHS, Russell FS, and Younge M (eds.) Advances inMarine Biology. 1–369.

Morgan MD (ed.) (1982) Ecology of Mysidacea. Hydrobiologia93: 1–222.

Nesler TP and Bergersen EP (eds.) (1991) Mysids in Fisheries:Hard Lessons from Headlong Introductions. American FisheriesSociety Symposium 9. Bethesda, Maryland: American Fisheries

Society.Riessen HP (1982) Pelagic water mites: Their life history

and seasonal distribution in the zooplankton community of a

Canadian lake. Archiv fur Hydrobiologie Supplement 62:

410–439.

Sprung M (1992) The other life: An account of present knowledge

of the larval phase of Dreissena polymorpha. In: Nalepa TF andSchloesser DW (eds.) Zebra mussels. Biology, Impacts andControl, pp. 39–54. Boca Raton: Lewis Publishers.

Sweetman JN and Smol JP (2006) Reconstructing fish populations

using Chaoborus (Diptera: Chaoboridae) remains – A review.Quaternary Science Reviews 25: 2013–2023.

Swift MC (1992) Prey capture by the 4 larval instars of Chaoboruscrystallinus. Limnology and Oceanography 37: 14–24.

Wissel B, Yan ND, and Ramcharan CW (2003) Predation andrefugia: Implications for Chaoborus abundance and species

composition. Freshwater Biology 48: 1421–1431.

Relevant Websites

www.jellyfish.iup.edu – Freshwater jellyfish. Web site maintained

by Dr Terry Peard, Professor of Biology, Indiana University ofPennsylvania, Indiana, PA. This site includes much information,

including video clips of freshwater jellyfish.

http://el.erdc.usace.army.mil/zebra/zmis/ – Zebra mussel informa-

tion. The US Army Corp of Engineers maintain this website‘zebra mussel information system’ with excellent information

on all aspects of zebra mussel biology.